1. Field of the Invention
This invention relates to the field of infrared sensing, and more particularly to a method and apparatus for calibrating an imaging sensor.
2. Description of the Related Art
Elemental infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having wavelengths of 1-15 .mu.m. To detect infrared radiation, these elemental detectors often use temperature sensitive pyroelectric crystals such as triglicine sulfate and lanthanum doped lead zirconate titanate. Such crystals exhibit spontaneous electrical polarization in response to incident infrared radiation which creates a potential drop across the electrodes of the detector. Elemental detectors may also be fabricated from materials which rely on photoconductive or photoemission properties to detect infrared radiation.
Arrays of such elemental detectors may be used to form thermal imaging systems. In real time thermal imaging systems such as forward looking infrared ("FLIR") imaging sensors, oscillating prism mirrors are used to scan radiation emitted by a source across a one-dimensional array of elemental detectors. When the elemental detectors are used in this manner, the temporal outputs of the detectors may be used to generate a two-dimensional representation of the image. In two-dimensional detector array imaging systems images such as those using staring detector arrays, the elemental detectors are used to produce free charge carriers which are then injected into a change coupled device ("CCD"). The output from the CCD is then processed by using time delay integration and parallel-to-serial scan conversion techniques.
Because each detector channel (i.e., the detector together with its coupling and amplifying electronics) in an imaging sensor often produces a different response to a given intensity of infrared radiation, it is often necessary to calibrate the detector channels so that a given infrared signal would produce approximately the same output at each channel. To provide for such calibration, it was often necessary to use an extended source emitting a uniform level of infrared radiation. When such an extended source was used, all the detectors would focus on the source during the calibration cycle while their outputs were measured. The outputs from the detector channels would then be compared so that the processing electronics could compensate for the differences in the electrical characteristics of the channels. As an alternative technique for calibrating a detector array, each of the elemental detectors were sequentially exposed to a constant intensity point source such as a scanned laser or star. After the outputs of each of the detector channels were measured, the processing electronics would determine the relative output variation of the detector channels so as to enable array calibration.
While the methods for calibrating the detector channels described above were somewhat effective, they often had several disadvantages. The alternative methods which used extended sources had to have a uniform distribution of intensity, a condition difficult to achieve in practice. Further, using constant intensity point sources for calibration was often inefficient in terms of calibration time as each individual detector element had to scan the same point source before the processing electronics could provide the necessary signal adjustment.